Is Iphone 4 8gig MD125LL unlocked?

I bought a iPhone 4 8gig at craigslist, and the model number is MD126LL and that seller told me it's a att iPhone(theres a ATT sim in the phone). But when i turn on and put my tmobile sim card in and it works, is this meaning this iPhone 4 is factory unlocked?

I bought a iPhone 4 8gig at craigslist, and the model number is MD126LL and that seller told me it's a att iPhone(theres a ATT sim in the phone). But when i turn on and put my tmobile sim card in and it works, is this meaning this iPhone 4 is factory unlocked?

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